Copper(II) Sequentially Loads onto the N-Terminal Amino Group of the Cellular Prion Protein before the Individual Octarepeats

The cellular prion protein (PrP^C) binds to Cu²⁺ ions <i>in vivo</i>, and a misfolded form of PrP^C is responsible for a range of transmissible spongiform encephalopathies. Recently, disruption of Cu²⁺ homeostasis in mice has been shown to impart resistance to scrapie infection. Using full-length PrP^C and model peptide fragments, we monitor the sequential loading of Cu²⁺ ions onto PrP^C using visible circular dichroism. We show the N-terminal amino group of PrP^C is not the principal binding site for Cu²⁺; however, surprisingly, it has an affinity for Cu²⁺ tighter than that of the individual octarepeat binding sites present within PrP^C. We re-evaluate what is understood about the sequential loading of Cu²⁺ onto the full-length protein and show for the first time that Cu²⁺ loads onto the N-terminal amino group before the single octarepeat binding sites